1. Field of the Invention
The present invention relates to loop radiating elements, and more particularly to loop radiating elements operating at microwave frequencies above a reflecting ground plane parallel to the plane of the loop.
2. Description of the Prior Art
Surveillance radars operating from orbiting satellite platforms generally do not cover targets near the nadir direction because of excessive ground clutter, but do cover an annular region from the so-called "nadir hole" to the horizon. For some surveillance modes horizontal polarization is greatly preferred over vertical polarization, and for this case an array of horizontal loops is an attractive alternative to switched orthogonal dipoles. The element pattern of a loop is doughnut-shaped with nulls along the loop axis.
Most loop antennas are for low frequency communication or navigation system applications. An Alford loop, as shown in FIG. 1, is a square configuration fed by a two-wire balanced line connected to a pair of terminals at its center. This loop has a convenient input impedance (approximately 80 ohms) and uses a transposition (cross-over) connection on one side of the feed terminals to achieve the proper unidirectional currents in its four radiating segments.
An Alford loop designed for microwave frequencies in printed circuit form is shown in FIG. 2 where the cross-over is achieved by printing two parts of the loop on opposite sides of the circuit board. An inherent assymetry problem with this loop model at microwave frequencies is that the null is off-axis. This occurs because the cross-over on one side causes the path lengths from the center feed terminals on that side to be longer than those on the other side. The result is that the phases and amplitudes of the currents in the radiating segments of the loop are unbalanced (unequal), causing imperfect cancellation on-axis. Attempts to relocate the feed terminal off-center to compensate for the cross-over have caused impedance matching problems and have been generally unsuccessful.